Intermediate System-to-Intermediate System (IS-IS) protocol is a dynamic routing protocol originally designed by the International Organization for Standardization (ISO) to operate on the connectionless network protocol (CLNP).
In IS-IS, all link states in the network forms a link state database (LSDB). There is at least one LSDB in each IS. The IS uses a shortest path first (SPF) algorithm and LSDB to generate its own routes. Each IS can generate a link State Protocol Data Unit (LSPDU) or link state packet (LSP) which contains all the link state information of the IS. Each IS collects all the LSPs in the local area to generate its own LSDB.
Graceful Restart (GR) is a mechanism that enables a device to forward data uninterruptedly when it performs an active/standby switchover or restarts a routing protocol. When a GR-capable device restarts a routing protocol, it notifies the event to its neighbors, which then maintain adjacencies and the routing information of the device within a specified interval. After the protocol is restarted, the device retrieves the information (topology, routing and session information maintained by the protocol supporting GR) from the neighbors and restores the state before reboot. During the restart process, no route flapping occurs and no forwarding path is changed. Thus, the system can operate continuously. IS-IS GR can ensure the service continuity of an IS-IS-enabled device such as a router during active/standby switchover or IS-IS restart.